1. Field of the Invention
The present invention relates to a toroidal continuous variable transmission with a speed change unit that continuously changes the rotation speed of an input disk according to a tilt angle of power rollers with respect to the input disk and output disk arranged opposite each other and transmits the changed revolution speed to the output disk.
2. Description of the Prior Art
The toroidal continuous variable transmission generally has a toroidal speed change unit 1 shown in FIG. 5. The toroidal speed change unit 1 of the toroidal continuous variable transmission comprises an input disk 2 and an output disk 3 arranged opposite each other; a pair of power rollers 4 (only one power roller is shown) that continuously change the revolution speed of the input disk 2 as their tilt angles with respect to the input and output disks 2, 3 change, and transmit the regulated revolution to the output disk 3; and a pair of trunnions 6 (only one of them is shown) each rotatably supporting the power rollers 4 and tiltable about a tilt shaft 5.
The trunnion 6 is normally at the neutral position for a certain transmission ratio. That is, the trunnion 6 is disposed at a position where the rotation axis A--A of the input disk 2 and output disk 3 crosses the rotation axis B--B of the power roller 4 (=neutral position). Speed change is effected by displacing the trunnion 6 in the axial direction of the tilt shaft 5 from the neutral position. As the trunnion 6 is displaced in the axial direction of the tilt shaft 5, it is pivoted about the tilt shaft 5 in a direction and at a speed that correspond to the direction and amount of the displacement to change the ratio between a radius described by the contact point of the input disk 2 and the power roller 4 and a radius described by the contact point of the output disk 3 and the power roller 4. The changed radius ratio causes the revolution speed of the output disk 3 to change relative to that of the input disk 2.
In the above toroidal continuous variable transmission, the power roller 4 is tilted by a control apparatus. A variety of control apparatuses have been known. An example control apparatus (Japan Patent Laid-Open No. 184262/1986), as shown in FIG. 5, comprises: a sleeve 11 slidably installed in a hole in a valve case 12; a spool 13 slidably fitted in the sleeve 11; a precess cam 18 moved together with the trunnion 6 to displace the spool 13 in the axial direction; a hydraulic cylinder 8 which, as the spool 13 and the sleeve 11 axially slide to change their positional relationship, is supplied with or drained of oil pressure to displace the trunnion 6 in the axial direction of the tilt shaft; a drive means 19 to displace the sleeve 11 in the axial direction; and a controller 20 to send a control signal representing a target transmission ratio to the drive means 19. There are various types of drive means 19. The drive means 19 shown in FIG. 5 uses a solenoid valve 19 to control the oil pressure acting on the end of the sleeve 11.
The solenoid valve 19 controls the magnitude of pressure Ps acting on the left end of the sleeve 11 according to the output signal from the controller 20. The sleeve 11 is shifted toward the right in FIG. 5 by the action of the pressure Ps. Because the sleeve 11 is urged toward the left by a return spring 15, the application of oil pressure through the solenoid valve 19 to the left end of the sleeve 11 causes the sleeve 11 to move to a position where the pressure Ps and the force of the return spring 15 balance each other.
In the above control apparatus, the controller 20 sends an output signal representing a target transmission ratio to the solenoid valve 19. As shown in the flow chart of FIG. 6, when the control apparatus starts, the main routine calculates the target transmission ratio e.sub.0 based on the speed change information (step 21). The controller 20 then calculates a duty according to the target transmission ratio e.sub.0 (step 22). The calculated duty is output to the solenoid valve 19 (step 23). The processing returns to the start of the main routine.
Next, the speed change operation performed by the toroidal continuous variable transmission is explained in the following. Let us first consider a case of shifting toward a speed-decrease side by referring to FIG. 5.
(1) A signal from the controller 20 activates the solenoid valve 19 to apply the pressure Ps to the left end of the sleeve 11, which is then moved toward the right from the position shown in FIG. 5. When the positional relation between the sleeve 11 and the spool 13 change, the Pd circuit and the PL circuit communicate with each other supplying the line pressure PL to a cylinder chamber 8b on the speed-decrease side from the oil pressure source. At the same time, the Pu circuit and the drain circuit communicate, draining the oil pressure from a cylinder chamber 8a on the speed-increase side to a tank. As a result, the relationship of circuit pressures becomes Pd&gt;Pu, which offsets the trunnion 6 downward causing the power roller 4 to start tilting about the tilt shaft 5 in the direction of arrow DOWN by the side slip force.
(2) As the power roller 4 tilts, the spool 13 is shifted toward the right in FIG. 5 by a distance that corresponds to a power roller's synthesized displacement--a combination of the power roller's displacement in the axial direction of the tilt shaft and its tilt angle--thus throttling the communication passage between the Pd circuit and the PL circuit and the communication passage between the Pu circuit and the drain circuit. When the positional relationship between the sleeve 11 and the spool 13 becomes neutral, the circuit pressures becomes Pd=Pu.
(3) In this state, however, the power roller 4 is still offset in the axial direction of the tilt shaft and thus continue tilting by the side slip force. This in turn causes the spool 13 to move to the right from the neutral position relative to the sleeve 11, opening the communication passage between the Pd circuit and the drain and the communication passage between the Pu circuit and the PL circuit. As a result, the pressure relation in the circuits becomes Pd&lt;Pu, moving the trunnion 6 upward, reducing the displacement of the power roller 4 in the axial direction of the tilt shaft, which in turn weakens the side slip force, reducing the pivoting or tilting speed.
(4) As the trunnion 6 repeats the vertical reciprocating motion with the neutral position at the center, the oscillation amplitude decreases progressively until the axial displacement of the power roller 4 is zero and the position of the spool 13 is neutral with respect to the sleeve 11, at which time the speed change operation is completed.
When the temperature of the working oil acting on the left end of the sleeve 11 changes, the viscosity of the oil varies greatly. In the conventional toroidal continuous variable transmission described above, the change in the viscosity of the working oil changes the output pressure characteristic of the solenoid valve 19, so that the sleeve 11 is controlled to a position different from that to which the controller 20 intends to move the sleeve 11. As a result, a desired transmission ratio cannot be obtained.
Furthermore, the components such as solenoid valve and return spring have variations in characteristic due to manufacture variations and changes with time, resulting in the transmission ratio being distributed with respect to the output signal from the controller to the solenoid valve and thus being unable to be controlled to an intended value.